Vaccinia virus contains a genome of more than 180,000 base pairs that encodes approximately 200 polypeptides. These genes are expressed in a coordinated fashion so that some polypeptides are made before and others after DNA replication. A large number of early and late genes have been transcriptionally or translationally mapped and some of each class have been sequenced. The regions preceding the coding segment are extremely rich in adenosine and thymidine nucleotides and differ substantially from eukaryotic consensus sequences. These sequences have been identified as promoter regions by in vivo and in vitro experiments. The regulatory signals specifying early or late expression are closely linked to the promoter. Evidence for the existance of virus-specific trans-acting transcriptional factors has been demonstrated by use of vaccinia virus infected cells for transient expression of recombinant plasmids. These factors have been demonstrated directly in extracts of infected cells.